Joseph's Box, By Suhayl Saadi

This is a novel of visceral, imaginative excess and, to quote Henry James, a large, loose, baggy monster. Joseph's Box is like Keats's snake goddess, Lamia: she contains so much colour, beauty and lust that she is almost unimaginable, too metaphorically strained for corporeal life. This novel, in attempting to be so many things, is in danger of becoming too diffuse.

It reaches for profundity through countless divergent lines of fecund thought, over-stretches, and never quite centres on anything. It maps an adventure that contains brilliant moments of drama and dream. Saadi is a writer of rare, raw, risk-taking talent: that much was evident in 2004's Psychoraag. But in Joseph's Box the set pieces of breathless and horrific action lack credibility, because they seem designed primarily to shock. Sometimes they soar, but occasionally they fall flat on the flabby cheeks of their own artifice.

The context is rich and varied: the contemporary Glasgow setting, Indian, Persian and Pakistani cultures, and mythologies of all kinds. For all its references to the "real" and to "magic", the genre of Joseph's Box is not magical realism, but a self-consciously multicultural adult fantasy. The characters are never sure of the events they witness or dream. They despise Glasgow and capitalist modernity, but they are hardly politicised critics. All think in a hazy and insipidly poetic manner – a free association which conjoins the fantastical with the mundane.

The novel sets up a racy adventure story, starting with the lead female Zuleikha fishing a box out of the Clyde. She is helped by lute-playing Alex, and their journey begins. Both are grieving and lonely, and quickly become unwitting picaresque heroes. What happens to them becomes increasingly trippy, as they open each successive box, and follow the cryptic clues inside. They are taken to a disused underground station, a church crypt, an old airfield, the wilds of Lincolnshire, a witch's cottage, and then across the world to places full of music, sex and terror – all richly, drawn in thick detail.

Saadi does not draw distinguishable world-views for his three main characters, which means that all think and speak in similar ways. In the midst of their standard English, they all deploy obscure medical terms, which can only be logical for the lead character, Zuleikha, a GP. Many Glaswegian novels have criticised the exclusivity of medical language, which serves to keep the patient under control: think of Alasdair Gray's Lanark, Janice Galloway's The Trick is to Keep Breathing, and James Kelman's How late it was, how late. Saadi, as a Glaswegian medic, puts the reader in the miserable position of the ignorant patient. Cyanotic? Occiput? Cardiotoxic? This can be obtrusive language, especially in the midst of a hippyish world which disowns materiality. It is a shame that the realms of scientific rationality and of erotic, artful imagination do not always coalesce.

The "magic" moves from the chilling to the silly: plants that die on the death of a child, memories conveyed through touch, ancient maps on paper made wet by the tears of the lute player. Such inexplicable processes are evocatively rendered, but to what end? Ghostly historical labyrinths abound, but as Umberto Eco's Foucault's Pendulum concludes, the problem with secrets is that the moment they are revealed, they seem little.

The characters seek solace in fantasies. Sex is a principle area of exploration, so there's lots of human meat packed in. Zuleikha chews over the idea that "human beings were sewers, great humping pigs with enormous dribbling snouts. Especially when it came to fucking. Anything was possible." Her lover Alex declaims that "death was shit, no matter how or when it arrived".

These people painfully measure out the littleness of their comprehension. Here is a typical excerpt from Zuleikha: "Several centuries of urbanisation have alienated us from our own countryside, she thought. And yet, not so very long ago, we all were either peasants or else lords of the land. Mother Earth, Pandora, formed from mud, reluctantly opens her celestial box. Fuck Freud, she thought. Or on second thoughts, don't." Is this comic? Is it profound? Can it be both crass and subtle? Such moments render this novel problematic, but overall it is a failure which is sporadically glorious.